Thermo-electric devices



May 7, 1968 F. J. WILKINS v THERMO-ELECTRIC DEVICES 2 Sheets-Sheet '2 Filed May 11, 1964 ig 6. 4f, i

I JAH/wwe If. J.W1LK1N5 m M w T A United States Patent Olice 3,381,368 Patented May 7, 1968 3,381,368 THERMO-ELECTRIC DEVICES Frederick John Wilkins, T eddington, Middlesex, England,

assigner to National Research Development Corporation, London, England, a corporation of Britain Filed May 11, 1964, Ser. No. 366,284 Claims priority, application Great Britain, May 10, 1963, 18,699/ 63 4 Claims. (Cl. 29-573) ABSTRACT F THE DISCLOSURE A lmulti-junction thermo-electric device has the junctions produced by coatings of discrete lengths of one electrically conductive material on a cont-inuous wire of another electrically conductive material, the wire being wound into a helix with two rows of junctions formed by the ends of the coatings. A heater of rod-like form is provided along one row and the helix may be a ilat helix with a second unheated rod at the other row of junctions. The heater may be a hairpin looped electric heater. Two such helices may be intermeshed with the heater common to both. To make the device, lthe wire is wound into a rflatended helix on a at-sided core, narrow spacers being interposed on one side, The un-spaced side is protected by cementing to a support and the unprotected part coated with the other conductive material by electroplating. Narrow insulating strips are then cemented on and the core is dissolved away, this also releasing the spacers. The Arodlike supports are then threaded in.

The present invention relates to thermo-electric device-s used in or useful for instruments of various kinds, and incorporating a plurality of small thermocouples in series whereby a correspondingly higher ou-tput voltage is obtained for a given temperature difference between the hot and cold junctions.

According to the invention a multi-junction thermoelectric device comprises a continuous length of an electrically conductive material, and coating Ithereon over discrete lengths of another electrically conductive material, the coating material being of substantially higher conductivity than the continuous length. Provided thickness and contact between t-he coatings and continuous length are adequate in conjunction with the higher conductivity of the coatings to ensure that the coated lengths behave thermo-electrically virtually as if they consisted only of the coating material, thermo-junctions are formed at both ends of every Ilength of coating and by suitably disposing t-he junctions so that alternate junctions can be brought to different temperatures, a series arrangement is achieved.

The materials of the continuous length and the coatings must be selected to satisfy the above requirements, but within this restriction they may be selected to give a high rather than a low thermal E.M.F. The required difference in conductivity is best satisfied by choosing a resistive alloy for the continuous length, for example, a copper nickel alloy such as constantan and a single metal of high conductivity for the coatings, -for example, silver, copper or iron. Copper is a preferred choice as silver is more liable to tarnishing while iron is of lower conductivity. Good contact and adequate thickness of copper are readily achieved 'by electroplating a thin resistance wire. Hereinafter constantan and copper will be assumed without the invention being limited thereto.

For many purposes such a-ther-mo-couple device may be combined with an electric heater when it serves as a converter, the output voltage being a function of the heating current. Such convertors are used, for example, for

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measuring alternating currents, the current or a known fraction of it serving to energise the heater, while the thermo-couple output is applied to an instrument such as a millivoltmeter.

In incorporating a thermo-couple series according to the invention into such a convertor the wire incorporating the junction is wound in a flat helix over two rod-like members spaced apart, the coated lengths of the wire and the spacing of the rod-like members being related to locate alternate junctions at the respective rod-like members. The rod-like members must not short-circuit the turns of the helix and if as will generally be the case, the rod-like members are of metal, the turns must be insulated from the rod-like members without causing any substantial temperature gradient. lf now the rod-like members are brought to different temperatures the desired convertor action is obtained. For many purposes one of the rod-like members can be an element in the form of a wire or hairpin loop with close parallel limbs which can be heated by the passage of an electric current while the other rod-like member is a stout wire or a bar which determines the temperatures 0f the cold junctions. For comparative purposes, however, both rod-like members may be single or hairpin loop heating elements thus forming a differential device which will allow, for example, a `direct current to be compared directly with an alternating current, by supplying one element with the one curren-t and the other element with the other. A development which is convenient for some purposes is to provide three rod-like elements one in the centre and the other two spaced from it and from one another and for the thermocouple series lto include two sets both wound in the form of a flat helix over the central rod-like member but only over a respective rod-like member to one side and the other of the central member, the junction at the central member alternating along it. With this construction the centre member will usually be a heating element while the other two will be cold wires or bars.

In greater detail a convertor may .be made as will now be described with reference to the accompanying diagrammatic drawings in which FIGURE 1 is a plan vie-w of a complete thermo-electric convertor, according to one form of the invention,

FIGURES 2, 3, 4 and 5 illustrate stages in the manufacture of the multiple thermo-junctions, and

FIGURES 6, 7 and 8 illustrate stages in the manufacture of another form of the invention.

Referring to FIGURE 1, the heater element 11 is a narrow loop of wire, secured at one end by a clamping block 12. The heater element is contacted by a number of thermo-junctions A1 A2 Ar AH, An 1An, the thermo-junctions being in the form of loops, each of which embraces both legs of the heater element. To each heated junction A, corresponds a cold junction B, as remote as practicable from the heating element. Each cold junction, likewise, forms a loop. All the cold junction loops on the left of FIGURE l embrace a copper bar 13, and those on the right a similar copper bar 14. The cold junctions are electrically insulated from the copper bars by paper folds, 15, 16. Bar 13 is kept in position by clamps 17, 18, and bar 114 by clamps 19, 20. The clamps are mounted on a baseplate (not shown). Desirably, the clamps and base plate are made of polymethylmethacrylate resin (Perspex). This is true also of clamp 12 for the heater leads 21, 22 and of clamp 23 for therm'ocouple leads 24, 25. The whole is mounted in a surrounding shielding box, 26, of material of high electrical and thermal conductivity, suitably copper foil, blackened on the inside. The copper bars 13, 14 are soldered to the box. The box Ais made with apertures large enough to allow the leads 21, 22, 24, 25 to pass through. Best results are obtained if the convertor is placed in an evacuated enclosure.

By way of example the heater element in one specific embodiment is of the order of 11/2 inches long (i.e. about 3 inches of wire) and consists of 0.002 inch diameter resistance wire such as the quarternary alloy Karma comprising 72% nickel, 22% chromium, 11/2% iron and the remainder aluminium, having a coating of a suitable adhesive such as Durofix, a solution of cellulose acetate in amyl acetate and acetone, which gives it additional stiffness. Any original insulation on the wire is retained.

Thermocouples are prepared as follows. A length of constantan wire 27, sutcient to make a predetermined number of thermojunctions, is wound on a core 28 of an electrically insulating material which can subsequently be dissolved away in a specific solvent; a very suitable material is polystyrene. One side of the core should be fiat and the flat should occupy nearly half the periphery. A very suitable form of core has the wedge section of narrow wedge angle. For the specific embodiment being considered, wire of 0.001 or even 0.0008 inch diameter is suitable and the polystyrene core has a width of about 5/16 inch land a wedge angle of about 3".v In order that a gap shall be left between the turns of wire and the polystyrene core, the thick side of the core is enclosed, up to about 1/3 the breadth, with a spacer of insulating material which will not dissolve in the solvent for the polystyrene, suitably a fold of shellac impregnated paper 16, desirably having a thickness of 0.0005 inch. When all the wire required has been wound on to the polystyrene core, the latter is cemented down to a convenient support 29 with polystyrene solution, as indicated in FIGURE 3, pressing against the wires on this closed side of the wedge core a strip 30 of fine copper foil. In a suitable electrolytic bath, by passing a current through the copper strip, the wire on the exposed side of the wedge core is plated with copper. There is such a large difference between the conductivities of copper and constantan that the copper plated portions of the wire act virtually as if they were solid copper. Thermo-junctions are therefore formed at the points 31, 32 (FIGURE 4) of each turn of the wire round the wedge core. After the plating process insulating strips which do not dissolve in the solvent for the polystyrene, suitably a mica strip 33 and a paper strip 34 are cemented to the assembly with a cement which also does not dissolve in the solvent for polystyrene, suitably shellac. The whole is then immersed in benzene which dissolves away the wedge strip and releases the wound wire, with the paper fold 16, the mica strip 33, and the paper strip 34, from the base 29. The paper strip serves to keep the loops at the correct spacing; the mica strip serves the same purpose and additionally facilitates handling.

Two sets of thermocouples, prepared as described above, are required for each convertor according to the present embodiment. The two are interleaved at right angles as shown in FIGURE 5. This step is faCiltated by coating the surfaces 35, thinly with petrolatum (Vase line). A heater element 11 is then inserted at 36 through all the loops of .the two sets of thermocouples. Copper bars 13, 14 are then threaded through the paper fold in each set. The copper bars may be about 1/16 inch wide and 0.02 inch thick. The clamps 17, 18, 19, 26 are then placed in a jig (not shown) and the copper bars, bearing the thermocouple sets yand heater, are placed in the clamps. Jig attachments pull sideways on the copper bars until the thermocouple loops (the hot junctions) make good contact with the heater element, and the clamps are than tightened. The jig may then be removed. The paper and mica strips are next removed by dissolving away the shellac with alcohol. The connections to the heater and the thermocouples are then clamped at 12 and 23 and soldered to the external leads. The copper foil box, 26 is then put round the assembly and soldered at 4 36, 37, 33, 39 to the ends of the copper bars 13, 14. This provides that the cold junctions, B1 B2 and so on, are kept as nearly as possible at the temperature of the surrounding copper shielding box 26.

5 In the present embodiment the number of hot junctions is 55, but there could be fewer or even more. A great advantage accruing from the use of so many couples is that a much smaller temperature rise can be accepted than with conventional Convertors, i.e. about 4 C. as

compared with about 200 C. This gives much better obedience to Newtons Law and hence better adherence to the desired square law relation between heater current and thermocouple output. A typical output of a conventional convertor would1 be 6 to 7 mv. as a maximum,

15 whereas the present embodiment according to the invention will give an output of 100 to 120 mv. It has a `maximum heater' current of the order of 70 ma. The heater resistance is about 40 ohms and the thermocouples have a resistance of 400 ohms. The specific output, i.e. millivolts per milliwatt input to the heater is substantially higher than that obtainable from the conventional single junction Convertors.

The present invention has a short response time, that is the time for the convertor to reach within 1 percent from its final reading, of about 4 seconds.

Except for the use of a differential convertor to compare a direct current with an alternating current, a direct current is generally only used in the heater for calibration. Convertors according to the present invention have a very low D.C. reversal, that is the change in output when the polarity of the heater when supplied with direct current is reversed, the value not exceeding one part in 105. Also the A.C./D.C transfer, is small and according to calculation in the present embodiment should not exceed one part in 105 for frequencies between about 25 c./s. and 5 kc./s.

The form of the invention now to be described with reference to `FIGURES 6 to 8 enables even more junctions to be included conveniently in a given space than that above described and also facilitates the production of a differential converter i.e. a converter with a heating element at each row of junctions, while still having advantages and properties of the same order as the first described embodiments.

In this form the heater wire 41, either single or bilar, is attached to one of the long edges of a rectangular piece 42 of thin mica, for example l inch by 1/16 inch by 0.001 inch. This mica is fixed Iby polystyrene cement to a piece 43 of polystyrene of the same length, greater width, say

4 times, and greater thickness say 0.010 inch, so that it is placed centrally lengthwise and so that the heater just protrudes beyond the edge of the polystyrene. A second piece 44 of thin mica of the same size as the first is attached to the same side of the polystyrene as the first piece but along the opposite edge, as shown in elevation in FIGURE 6. If a differential convertor is being made this mica also carries a heater.

A piece 45 of highly conductive, suitably copper, foil suitably 0.0005 inch thick and approximately 1/16 inch wide is placed on the surface of the polystyrene 43 so that it lies between the two pieces 42, 44 of mica. It is lightly fixed to the polystyrene by polystyrene glue at its ends. This copper serves to make electrical contact to the constantan wire afterwards wound on.

55 Two Ifurther pieces 46, 47 of mica longer, wider and stouter than the pieces 42, 44, say 11/4 inch by 1%@ inch by 0.005 inch yare glued to the other surface of the polystyrene 43 resulting in a section shown in FIGURE 7. These pieces 46, 47 of mica stiften the polystyrene, so that the wire can be wound around it.

Constantan wire 48, suitably 0.000-8 inch diameter, is wound under the maximum possible tension on the composite strip. A spacing of 0.006 inch between adjacent turns is easily lachieved but can be made longer or even smaller according to requirements. After winding the turns are attached to both pieces 42, 44 of mica by means of a thin solution of Duroiix. The polystyrene 43 is bonded to further piece of polystyrene so that the two pieces 42, 44 of mica and the copper foil 45 are sandwiched between the two. The exposed part of the constantan wire 48 is then plated with copper. It will be noted that the plating will extend right up to the heater 41.

After plating a third piece 49 of thin mica of the same size as the pieces 42, 44 is introduced. It is inserted between the pieces 46, 47 of mica, and the plated wire, and is placed over the gap between these two pieces of mica. The plated wires are fixed to this piece 49 by means of a thin solution of Duroiix, The plated wires are attached to the heater or heaters by running a thin solution of Durox along its length. The assembly is then immersed in benzene and the polystyrene dissolved away. At this stage the stilfening pieces 46, 47 of mica and the copper strip 45 are free and can be removed. This leaves the heater or heaters attached to the wire winding with the wire on one side of the assembly plated. FIGURE 8 illustrates the assembly in section.

It will be understood that the use of mica is not essential for the strips 42, 44, 46 and 47. What matters is that they should be of insulating material of adequate strength which is not soluble in the solvent used for dissolving the strip 43 and can be secured by a cement which will dissolve in the solvent.

To complete a single heater type of convertor the cold junctions a-re embedded in a groove in the copper base of a copper housing that can be evacuated. The junctions are insulated from the copper base by means of thin paper strip, say 0.0005 inch thick. For a differential type with -a heater at both sets of junctions the complete assembly is supported so that it hangs freely inside an evacuated copper housing. In this second form of the invention, with a spacing of 0.006 inch, 160 couples can be included in the same lengths as 55 in the case of the iirst described form, thus enabling an output of up to 500 millivolts to =be obtained.

The convertor according to the invention has applications as a transfer device in accurate A.C. measurements. But it also has possibilities for computers. The most obvious use is in obtaining more accurate results in current measurements at audio and high frequencies and in the operation of high 4frequency watt meters. The convertor can be used at high megacycle frequencies since the capacitance on the input side is only a few pf., and even better performance might be obtained by substituting a single straight through heater wire for a bilar heater.

What I claim is:

1. A method of making a thermo-electric device which includes the steps of winding a length of alloy resistance wire in a helix over a core of electrical insulating material soluble in a specific solvent and a thin spacer insoluble in said solvent, the core having at least one at side and the spacer extending over a narrow portion only of the at side so that over the tlat side a gap is left Ibetween the turns of wire and the core, cementing the wound core with a cement soluble in said solvent to an electrically insulating support and at the same time pressing against the wires a strip of high electrical conductivity which leaves substantially half the length of each turn of wire including the portion extending over the ilat side of the core exposed, immersing the whole in an electrolytic bath, passing an electric current through the strip to electroplate the exposed parts of the wire with metal of the strip, cementing insulating strips which do not dissolve in said solvent on the exposed turns of wire with a cement which does not dissolve in said solvent, and immersing the whole in said solvent until the core and the cement holding the wire to the insulating support are dissolved away.

2. A method of making a thermoelectric device which comprises twice performing the steps o'f claim 1 using -a core of narrow angle wedge section with the thin spacer at the base end of the wedge, and after dissolving away the cores and cement holding the two Wound and plated wires at right angles with the angled ends interleaved, passing a central rod-like member through all the loops of the interleaved windings, passing a rod like member through all the loops of each of the windings near the base end, pulling on the last mentioned rod-like members to bring the loops at the angled ends of the windings into good contact with the central rod-like member, clamping the whole, and removing the cemented insulating strips.

3. A method of making a thermo-electric device which includes the steps of securing a rodlike electrical heater element along one edge of a first narrow strip of insulating material with a cement, the cement and the insulating material being insoluble in a specific solvent, securing the narrow strip Iby a cement soluble in said solvent to one side of -a wider strip of another insulating material which is soluble in said solvent, with the heater element located at one edge of the wider strip, similarly securing a second narrow strip of the lirst mentioned insulating material to the -same side of said wider strip as said irst narrow strip with one edge located at the edge of said wider strip which is opposite so that at which said heater element is located, securing a narrow strip of foil of conductive metal by local applications of cement soluble in said solvent to the saine side of said wider strip between the first and second narrow strips, securing by cement soluble in said solvent stiiening strips to the other side of said wide strip, leaving a narrow clear width along the centre of that side of the strip, winding under tension a thin wire of electrically resistive material into a closely spaced helix around the structure lof said heater element, narrow strips, wider strip foil and stitfeners, and thereby into conductive contact with the foil, cementing the turns of the wire to said first and second narrow strips with a cement insoluble in said solvent, bonding a covering of insulating material over the parts of the turns of the Winding on the side of said wide strip to which said first and second narr-ow strips are cemented, immersing the whole in a bath of electrolyte and electro-plating the exposed parts of the turns with the metal of said -foil strip, introducing a third narrow strip of insulating material insoluble in said solvent between the plated part of the turns and the wider strip along the narrow clear width of said wider strip, cementing this third strip to the turns of wire with a cement insoluble in said solvent, immersing the Whole assembly in a Ibath of said solvent until said wider strip, said covering and said soluble cement are dissolved away and said foil stri-p and stiifening strips are released, and removing said foil strips and stiffening strips.

4. A method as set forth in claim 3 also including the steps of securing a second rod-like heating element along that edge of said second narrow strip which is located at the edge of said wider strip when the second narrow strip has been cemented to the wider strip, and after electroplating, cementing said second heater element to said turns with a cement insoluble in said solvent.

References Cited UNITED STATES PATENTS 2,427,508 `9/ 1947 Raspet 136-201 X 2,580,293 -12/1951 Gier 136-233 X 2,807,657 9/ 1957 Jenkins 29-573 WILLIAM I. BROOKS, Primary Examiner. 

